1. Field of the Invention
The present invention relates to an improved method for the preparation of vanadium/phosphorus mixed oxide catalysts, using an additive such as dimethyl sulfoxide during the preparation, the catalyst having special utility in the production of maleic anhydride.
2. Description of the Prior Art
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, with molecular oxygen or oxygen containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve reducing a pentavalent vanadium compound, and if desired, promoter element compounds under conditions which will provide or maintain vanadium in a valence state below +5 to form catalyst precursors which are recovered and calcined.
Hydrogen chloride has been used as a reducing agent for vanadium compounds where vanadium has a +5 valence. The use of gaseous HCl as a reducing agent is disclosed in U.S. Pat. No. 4,002,650 where the vanadium and phosphorus components are reacted in an aqueous solution. The use of gaseous HCl as a reducing agent for vanadium compounds such as V.sub.2 O.sub.5 is also described in U.S. Pat. No. 4,043,943 where the vanadium and phosphorus components are reacted in liquid organic medium.
U.S. Pat. No. 5,137,860 provides a comprehensive description of the prior art in this area. The patent shows the use of organic reducing agents as well as hydrogen chloride and teaches the use of activation procedures whereby the catalyst precursor is contacted at prescribed conditions with oxygen and steam mixtures and finally with a non-oxidizing steam atmosphere to produce an active catalyst.
U.S. Pat. No. 4,569,925 describes the preparation of vanadium/phosphorus mixed oxide catalysts by an organic solution method using anhydrous hydrogen chloride as an agent for the solubilization of the vanadium component, and teaches an activation procedure whereby the catalyst precursor is contacted not with air alone but with a mixture of air and a hydrocarbon such as methane, ethane, propane, butane and the like.
U.S. Pat. Nos. 4,116,868 and 4,149,992 describe the preparation of vanadium/phosphorus catalysts wherein a surfactant is employed in the preparation procedure. Among the many surfactants suggested are dialkyl sulfoxides having a C.sub.10 -C.sub.18 alkyl group.
There are problems associated with the use of hydrogen chloride in the preparation of catalysts. The corrosion problems are obvious. Even after calcination to prepare the catalyst, residual chloride remains in the catalyst. The chloride is generally removed during the catalyst activation period in the reactor or in a separate step outside the reactor, but chloride release from the solid catalyst into the reactor and the downstream equipment in the process is undesirable. The main problems are: possible equipment corrosion, product loss during chloride liberation and increased waste disposal. It would be advantageous to provide a method for the preparation of VPO catalysts wherein the use of chloride materials could be substantially or completely avoided.
The synthesis of VPO catalysts can be carried out both in aqueous and in organic solvent media. Anhydrous conditions are preferred in the organic solvent method. The synthesis in organic solvents is presently the preferred method due to the better performance of the catalyst. This is attributed to greater surface areas of the catalyst when prepared in organic solvent than in aqueous media (G. J. Hutchings, Applied Catalysis, 72(1991), 1-32 and references therein).
In the organic solvent method typically employing isobutanol, anhydrous HCl has been used as reducing agent for the V.sub.2 O.sub.5. Other reducing agents have been used such as oxalic acid or organic alcohols such as allyl alcohol, benzyl alcohol and isobutanol which can be both the solvent and reducing agent. With HCl, the V.sub.2 O.sub.5 is converted to an IBA (isobutyl alcohol) soluble material (VOCl.sub.2) prior to the addition of phosphoric acid. In the absence of HCl, the V.sub.2 O.sub.5 is not solubilized and the formation of the VPO catalyst is done heterogeneously on the suspended V.sub.2 O.sub.5 in the organic solvent. The use of HCl has produced excellent catalysts but the residual chloride in the catalyst results in a chloride release during catalyst activation which is undesirable. This difficulty can be overcome by removing the chlorides through an additional step during the catalyst manufacturing.
Synthesis of VPO catalyst without HCl eliminates the use of this corrosive gas during production, reduces costs associated with waste disposal and eliminates the need to remove the residual chloride from the catalyst.